(1) Field of the Invention
This invention relates to the forming of trenches in a silicon substrate for shallow trench isolation and more specifically to using a dry isotropic etching step to form a rounded bottom of the trench and avoid sharp corners at the intersection of the sidewalls and bottom of the trench.
(2) Description of the Related Art
Shallow isolation trenches are frequently used for device isolation in integrated circuit wafers. These trenches are formed using dry isotropic etching and result in sharp corners 16 at the intersection between the bottom and the sidewalls of the trenches, see FIG. 1. These sharp corners cause stresses in the wafer when the trenches are filled with a dielectric and can result in wafer damage.
Polymer deposition is used to produce a rounded bottom of the trench and avoid the problem of the sharp corners. The use of polymer deposition, however, produces undesirable sloped edges of the trench. As dimensions of the trenches become smaller a steep trench profile is required. The use of the polymer deposition also reduces the mean time between chamber cleaning which adds to the cost of the use of trenches for isolation.
U.S. Pat. No. 4,855,017 to Douglas describes methods for etching trenches using reactive ion etching in a single wafer etcher. This methods uses passivation of the sidewall by passivation on a molecular scale to control the profile of the trench.
U.S. Pat. No. 4,729,815 to Leung describes a method of etching trenches with rounded top corners and rounded bottom corners. This method uses CHF.sub.3 as an etchant and the DC bias is adjusted differently for three different steps to provide profile control.
U.S. Pat. No. 5,068,202 to Crotti et al. describes a method of forming trenches using reactive ion etching of an ONO multilayer through a mask to define active areas of the substrate. A successive deposition of a conformable TEOS oxide followed by a blanket reactive ion etch leaves tapered oxide spacers on the flanks of the ONO multilayer. Isotropic plasma etching is then used to form a round profile trench with a rounded bottom and sloping sides.
U.S. Pat. No. 4,639,288 to Price et al. describes a method of forming trenches where the top part of the trench is wider than the lower part of the trench resulting in a tapered sidewall profile.
U.S. Pat. No. 5,651,858 to Lin describes a method of forming trenches having a tapered profile using a NF.sub.3 /HBr plasma etch.
U.S. Pat. No. 5,605,603 to Grimard et al. describes a method of forming deep, narrow, uniform trenches in a silicon substrate. The method uses a plasma generated from HBr, NF.sub.3, and O.sub.2.
U.S. Pat. No. 5,258,332 to Horioka et al. describes a method of forming rounded trench corners using chemical dry etching and a mixture of fluorine and oxygen.
This invention describes a method of forming round bottoms of a trench using a dry highly isotropic reactive ion etch.